Great suggestion. This is something we would love to do in the reports & analytics (SiteCatalyst) UI and are targeting for future work. Also, as you likely know, this can already be done in ad hoc analysis (Discover).
We use Discover a lot, but the majority of our users don't. And some mistakenly interpret "instances" as "page views." We recently discovered an issue where users' correlation reports include video milestones, as those props are being passed on the video call (in order to pass the data into an evar thru a processing rule). It'd be great if the end user could choose which metric to correlate.
